The present invention relates to a handover method, a base station, a mobile station, and a mobile communication system according to code division multiple access (CDMA).
In a mobile communication system, an exchange station (a base-station controlling station) is connected to base stations. Further, a mobile station may move during communication from the service area of a base station to the service area of another base station. At this time, a handover technique is employed for the mobile station to switch the base station to another without interrupting the communication of the mobile station. Therefore, it is desirable to improve the efficiency of the handover technique.
FIG. 12 shows an example of a handover technique. A mobile communication system includes base stations 1011, 1012, and the like, an exchange station 102, and a mobile station 103. The mobile station 103 communicates with a fixed station or another mobile station through the base station 1011, while the mobile station moves toward the service area of the base station 1012 as indicated with an arrow mark. At this time, the exchange station 102 and base station 1011, 1012 transmit and receive control information to hand over the mobile station 103 from the base station 1011 to the base station 1012 so that the mobile station 103 may continue the communication.
The above described stations to carry out the handover may be part of a CDMA mobile communication system. If the operating frequencies f1, f2 of the base stations 1011, 1012 are equal to each other, it is possible to achieve a soft handover by switching the base stations 1011, 1012 from one to another while maintaining the communication of the mobile station 103 through both the base stations 1011, 1012. If the operating frequencies f1, f2 are different from each other, a hard handover is required since the reception frequency of the mobile station 103 must be changed.
When being handed over from the base station 1011 to the base station 1012 in a CDMA mobile communication system, the mobile station 103 finds a time difference between a traffic channel used to communicate with the base station 1011 and a perch channel of the base station 1012, and then informs the base station 1011 of the time difference. The base station 1011 then informs the base station 1012 of the time difference through the exchange station 102. According to the time difference, the base station 1012 adjusts transmission-reception timing with respect to the mobile station 103 and starts to communicate with the mobile station 103.
In order to perform the soft handover without interrupting the communication of the mobile station 103, the base station 1012 must obtain a long code phase of the mobile station 103. The mobile station 103 has a long code period of 29xc3x9772 frames, and thus the base station 1012 is unable to automatically obtain the long code period. Accordingly, the mobile station 103 measures a phase difference between the perch channel of the base station 1012 and an outgoing traffic channel.
FIG. 13 shows an example of a signal format employed by perch channels. In FIG. 13, (a) shows a system frame having a period of 36,864 ms, (b) is a radio frame having a period of 10 ms, (c) is a 0.625-ms radio slot, (d) is a first perch channel signal in the radio slot having a pilot PL, data, and a long code mask symbol LM, and (e) is a second perch channel signal in the radio slot having only a long code mask signal LM. In FIG. 13, (f) and (g) show the signal format and long code of the first perch channel, while (h) and (i) show the signal format and group short code of the second perch channel. The time axis of (d) and (e) differs from that of (f) to (i).
One radio frame (b) consists of 16 radio slots, and one super frame (a) consists of 72 radio frames. The first and second perch channels are each made of slot periods. The first perch channel signal of (d) and (f) is spread according to the long code of (g). The long code mask symbol LM of the second perch channel signal of (e) and (h) is spread according to the group short code of (i).
FIG. 14 shows parts of a perch channel transmitter for a base station. Reference numeral 111 designates an antenna, 112, 113 transmitters, 114-116 spreading units, 117 a long code generator, 118 a common short code generator, 119 a group short code generator, and Swa, SWc switches.
The spreading unit 115 spreads data for the first perch channel according to a common short code from the common short code generator 118. The switch Swa is closed except during the period of the long code mask symbol LM (FIG. 13). This enables the spreading unit 114 to spread the data from the spreading unit 115 according to a long code from the long code generator 117. The transmitter 112 modulates the data from the spreading unit 114 into a signal having a transmission frequency, which is then transmitted from the antenna 111.
The switches SWb and SWc are closed in response to the long code mask symbol LM. This enables the spreading unit 116 to spread the data for the second perch channel according to a group short code provided by the group short code generator 119 through the switch SWb. The data from the spreading unit 116 is transferred to the transmitter 113 through the switch SWc and is modulated into a signal having the transmission frequency. The signal is transmitted from the antenna 111 together with the signal of the first perch channel.
The mobile station receives signals from the perch channels of peripheral base stations, despreads the signals according to the common short code, memorizes timing having a high correlative value, uses the timing to despread the signals according to all group short codes used for the second perch channels, and selects a group short code having a high correlative value. For example, a group short code involves 16 long codes, and 16 group short codes are prepared, so that 256 long codes are divided into 16 groups. Sixteen long codes belonging to the group short code having the high correlative value are used to despread the received data, and a long code having a high correlative value is found. The found long code is used to despread the received data and carry out a reception process.
Referring back to FIG. 12, if adjacent base stations 1011, 1012 employ the same frequency, a single frequency handover operation is carried out. In this case, the mobile station 103 may have a single receiver. The mobile station 103 is provided with a despreading unit for a traffic channel and a despreading unit for a perch channel. This enables the mobile station 103 to select a perch channel having the highest reception level during the handover process. If the adjacent base stations 1011, 1012 employ different frequencies, a different frequency handover operation is carried out. In this case, the mobile station 103 is provided with a receiver for the traffic channel frequency and a receiver for the perch channel frequency.
Providing the mobile station 103 with the two receivers increases the size, power consumption, and cost of the mobile station. In order to cope with this problem, it has been proposed to provide the mobile station with a single receiver and to switch the reception frequency of the receiver in time division to receive signals from both the traffic and perch channels.
In this case, each base station transmits data through a traffic channel in a compression mode as shown in FIG. 15. In the figure, (a) shows normal frames #1, #2, and the like transmitted in a normal mode at a rate of, for example, 8 kbps, (b) shows compressed frames that are made from the normal frames and are transmitted in a compression mode at a rate of, for example, 16 kbps. In other words, the compression mode temporally compresses two normal frames into a single frame to form a spare frame and transmits the compressed and spare frames with transmission power that is double the transmission power of the normal mode.
As shown in (c) of FIG. 15, the mobile station alternately receives signals from a traffic channel TRF in a period ta and a perch channel PER in a period tb that corresponds to the spare frame. As shown in (d) of FIG. 15, the single receiver of the mobile station is switched between frequencies f1 and f2 to alternately receive signals from the traffic channel TRF and perch channel PER. The signal received from the traffic channel TRF is decompressed two times to recover the original data, on which a reception process is carried out. During the period tb, a signal from the perch channel PER is received.
To carry out the handover, the mobile station must find a time difference between a present traffic channel and a perch channel of a destination base station accurately, for example, within a predetermined chip timing. At this time, a shift of a frame between the present traffic channel and the perch channel is detected by extracting a system frame number contained in a frame received from the perch channel. The head of each frame is then detected according to, for example, a synchronous signal. It is possible, therefore, to detect a shift between the head of a frame from the traffic channel and the head of a frame from the perch channel. Thus, time difference information including the frame shift and head shift is provided to the destination base station through the exchange station.
In practice, however, the perch channel adds an error detection code for every two frames including a radio frame ((b) of FIG. 13) that contains a system frame number and then interleaves the frames. A unit in which the error detection code adding process, an error correction coding process and the interleaving process are carried out is called a signal unit.
The mobile station receives the signal unit, deinterleaves the signal unit, carries out an error correction decoding process on the signal unit, detects any error, and identifies the system frame number of the signal unit. In other words, the mobile station must receive two frames of a signal unit from the perch channel. If the perch channel transmits the frames in a normal mode, the mobile station can receive only one frame in the reception period tb from the perch channel PER, i.e., the spare period made in the compression mode of (b) of FIG. 15. Therefore, the mobile station is unable to identify the system frame number.
To cope with this problem, it is possible to form a two-frame spare space so that the mobile station may receive two frames from the perch channel. A traffic channel presently communicating with the mobile station and a perch channel of a destination base station are asynchronous with each other. Thus, frames from the perch channel may be shifted as shown in either (e) or (f) of FIG. 15 at some time. In other words, in order to receive two frames from the perch channel in the asynchronous state, a single two-frame spare space may be insufficient. Therefore, a four-frame spare space is needed as shown in (g) of FIG. 15.
An object of the present invention is to reliably receive data from a perch channel without increasing a delay during a handout operation.
These and other objects are met by a handover method according to the present invention for a CDMA mobile communication system including base stations and the like, and a mobile station. The method includes that each base station compress frames having a number which is at least double the number of frames contained in a normal-mode signal unit into a compressed signal unit. Further, to repeatedly and continuously transmit the compressed signal unit providing the mobile station a reception period to receive frames equivalent to the normal-mode signal unit from a perch channel. In a period of, for example, four normal frames, the mobile station can reliably receive two frames from a perch channel as well as four compressed frames from a traffic channel.
The mobile station switching between receiving the compressed signal unit from a traffic channel within a normal-mode unit period corresponding to the normal-mode signal unit and receiving frames equivalent to the normal-mode signal unit from the perch channel within the normal-mode unit period.
The handover method also includes, while the base station is in the compression mode of transmission, alternately receiving frequencies at intervals of the normal-mode unit period according to frames received from the perch channel. Further, receiving frames alternately from the traffic and perch channels, and extracting a system frame, number from the frames received from the perch channel. If a signal unit consists of two frames, the mobile station can receive two frames from a perch channel to extract a system frame number therefrom.
The handover method also may include receiving frames alternately from the traffic and perch channels. However, the mobile station is unable to receive a given frame from the head thereof, the mobile station receives a first signal unit containing the second half of the given frame during a first unit period from the traffic channel. Further, receiving a second signal unit containing the first half of the given frame during a second unit period to recover the original state of the given frame by combining the first and second halves thereof.
According to the present invention, a base station is also provided for carrying out CDMA communication with a mobile station. The base station includes a data processor for temporally compressing frames to a number that is at least double the number of frames contained in a normal-mode signal unit into a compressed signal unit. The base station also includes memory for temporarily storing the compressed signal unit. A transmission controller is also included for transmitting the compressed signal unit, reading the compressed signal unit out of the memory and transmitting the read compressed signal unit.
According to the present invention, a mobile station is also provided for carrying out CDMA communication with a base station. The mobile station includes a receiver for switching between a traffic-channel reception frequency and a perch-channel reception frequency. This enables signals to be received alternately from the traffic and perch channels when the base station transmits signals through the traffic channel in a compression mode. A reception controller is also included for detecting the head of a frame received from the perch channel and then switching the traffic- and perch-channel reception frequencies from one to another at the detected frame head at intervals of a normal-mode unit period.
The mobile station also may include a despreading unit for switching a despreading code for the traffic channel and a despreading code for the perch channel in synchronization with the switching of the traffic- and perch-channel reception frequencies in the receiver.
The mobile station may also include a reception controller for shifting the timing of the switching of the traffic-and-perch-channel reception frequencies in the receiver by a frame of the perch channel so that a system frame number may be extracted.
The mobile station may further include a memory for temporarily storing frames received from the traffic channel when the receiver switches the traffic- and perch-channel reception frequencies from one to another. A read controller also may be included for reading each frame out of the memory from the head thereof. If unable to read any frame from the head thereof, reading the frame lastly and combining divided parts thereof together.
According to the present invention, a mobile communication system is provided including distributed base stations and a mobile station that carries out CDMA communication with the base stations. The base stations each compress frames having a number that is at least double the number of frames contained in a normal-mode signal unit to form a compressed signal unit. Further, repeatedly and continuously transmitting the compressed signal unit. The mobile station including a receiver for switching a traffic-channel reception frequency and a perch-channel reception frequency from one to another. A reception controller also included in the mobile station for detecting the head of a frame received from the perch channel and switching the traffic- and perch-channel reception frequencies from one to another at the detected frame head at intervals of a normal-mode unit period corresponding to the normal-mode signal unit.